1. Field of the Invention
The present invention relates to a high-speed cell searching apparatus and method for a communication system.
2. Background of the Related Art
In a direct sequence code division multiple access (DS/CDMA) communication system, a receiver is first required to synchronize PN sequences prior to detection of data. Mostly, the PN sequence synchronization is performed by two steps of code acquisition and code tracking in sequence.
Here, regarding the code acquisition, many researches have been made for a cell synchronous type system such as the existing IS-95 system whereby the whole cells share timing information using an external reference signal.
The most basic code acquisition-technique proposed up to now is a serial search. This method has an advantage that its implementation is not complex, but has a disadvantage that since the acquisition time is directly in proportion to the period of the PN sequence being used, it takes quite a long acquisition time in case that the period of the PN sequence is long.
Meanwhile, a parallel search has been proposed to acquire the PN sequences having a long period. However, this parallel search has a disadvantage that its implementation becomes complicated in proportion to the reduced acquisition time.
Ultimately, a hybrid method, which properly sacrifices and complements the acquisition speed and the complexity of implementation by adequately combining the serial search and the parallel search for the high-speed acquisition of the PN sequences, has been proposed, and another high-speed acquisition method based on state estimation of a shift register generator (SRG) has been proposed as well.
As the acquisition technique based on the state estimation of the SRG, there is a rapid acquisition by sequential estimation. This technique sequentially performs a hard detection of the received PN sequences a large number of times, carries them as temporary register state values of the receiving part SRG, and then finally decides whether or not it is synchronized through a confirming process. This technique has an advantage that it can greatly reduce the acquisition time without increasing the implementation complexity in comparison to the serial search in theory, but has a disadvantage that its performance abruptly deteriorates in case that it is based on a coherent detection of the PN sequences and has a low signal-to-noise ratio (SNR), and thus it is improper to apply it in a general code division multiple access (CDMA) environment.
Because of this, an acquisition technique based on a new state estimation named a distributed sample acquisition (DSA) technique has been proposed to reduce the acquisition time of the long-period PN sequences in the conventional cell synchronous type system.
According to this DSA technique, the transmitting part generates an igniter sequence having a relatively short period, and transmits the igniter sequence with state samples of the long-period PN sequences being carried thereon. The receiving part first acquires the igniter sequence, demodulates the state samples carried thereon, and corrects the SRG state of the receiver through a comparison-correction circuit at every accurate time point, so that the synchronization of the PN sequences is performed.
Since a pair of SRGs having the same structure and provided in the transmitting/receiving part perform the synchronization by carrying the same state values, i.e., L values stored in the SRG having the length of L, they can transfer the state samples of the transmitting part SRG to the receiving part using the DSA technique. As a result, this technique can acquire the PN sequence much faster than the existing technique that performs the synchronization at the time point where the maximum correlation value for the PN sequences is detected.
The high-speed acquisition using the DSA technique as described above has been limitedly applied to the cell synchronous type DS/CDMA system.
However, in the present environment that the DS/CDMA technique is expected to be applied to a cell asynchronous type system that does not depend on an external reference signal such as an international mobile telecommunication-2000 (IMT-2000), it is required that the DSA technique for more rapidly acquiring the PN sequence is extensively applied to the cell asynchronous type DS/CDMA system which is much more complicated in timing synchronization in comparison to the cell synchronous DS/CDMA system.
Meanwhile, the DS/CDMA cellular system for a mobile radio communication service such as the IMT-2000 to be developed is briefly classified into a cell asynchronous system and a cell synchronous system according as a reference timing between cells is provided or not.
In the cell synchronous system, a respective cell uses a sequence depending on an external reference timing providing resource such as a global positioning system (GSP) and obtained by differently shifting the phase of a single pseudo noise sequence as its scrambling sequence.
On the contrary, the cell asynchronous system uses a sequence differently given to the respective cell irrespective of the external reference timing as its scrambling sequence. For instance, in case of the asynchronous cellular system composed of 512 cells, 512 long-period scrambling sequences are required. For this, the cell asynchronous system such as the next-generation IMT-2000 differently combines the sequences generated using a plurality of (for example, two) fixed shift register generators (SRGs), and produces 512 different scrambling sequences.
The cell asynchronous system wherein the respective cell uses one among the 512 sequences performs a scrambling sequence acquisition for the sequence synchronization. In the scrambling sequence acquisition, a simple search for the frame timing required in the cell synchronous system, and discrimination of the sequence itself used in the present cell is performed as well.
According to the existing cell synchronous system, the serial search, parallel search, and hybrid method, which properly sacrifices and complements the acquisition speed and the complexity of implementation by adequately combining the serial search and the parallel search, have been used for performing the synchronization through the chip correlation of the sequences to acquire the scrambling sequence. Meanwhile, in case of the cell asynchronous system, a three-step cell searching method based on a method of removing a comma-free code and hierarchical uncertainty has been used.
The cell synchronous system using the serial search, parallel search, and hybrid method performs the synchronization by discriminating the timing representing the maximum correlation value obtaining by correlating the received scrambling sequence and the scrambling sequence generated by a mobile station. However, since the cell asynchronous system using the three-step cell searching method identifies both the kind and timing of the scrambling sequence, it should perform an additional synchronizing code transmission and more complicated signal process.
Hereinafter, the cell asynchronous system using the three-step cell searching method will be explained in detail, taking the W-CDMA system of IMT-2000 as an example, with reference to the channel structure and timing as illustrated in FIG. 1.
Respective 512 cells (i.e., base stations) transmit scrambling sequences of the corresponding cells through primary common pilot channels (P-CPICH), and transmit in parallel a primary synchronization code (PSC) which is commonly used by the 512 cells and secondary synchronization codes (SSC) which are peculiarly used by respective cell (i.e., base station) groups that the cells belong to.
Symbols transmitted through the SCH are transmitted, crossing in time with symbols of a broadcasting channel (BCH) transmitted through the primary common control physical channel (P-CCPCH) of the respective cells, in the transmission ratio of 1:9. Specifically, the primary synchronization code (PSC) and the secondary synchronization code (SSC) are simultaneously transmitted in a 256-chip section which is the first symbol transmission time point for each slot.
The mobile station (MS), at the first cell searching step, searches a slot boundary by searching the time point where a matched filter for the primary synchronization code (PSC) produces the maximum output value. At the second step, the mobile station identifies the frame boundary and cell (i.e., base station) group by decoding 15 secondary synchronization codes (SSC) transmitted for a one-frame length. At the third step, the mobile station searches the sequence representing the maximum output by correlating in parallel the scrambling sequences of the cells, which belong to the cell (i.e., base station) group searched at the previous step, with the received P-CPICH signals to complete the cell search. In the three-step cell searching method, the first and third steps relates to the process of reducing the uncertainty of the timing and the kind of the sequence through the correlation of the sequences well known in the art, and the second step is the actually peculiar process of reducing quite a large amount of uncertainty by the comma-free encoding and decoding.
Meanwhile, in order to effect a high-speed acquisition of the scrambling sequences, the distributed sample acquisition (DSA), which is quite different from the existing methods as described above, has also been proposed.
According to the DSA technique, in order to acquire the scrambling sequences at a high speed, the base station (i.e., cell) first transmits state samples of the SRG which produces the long-period scrambling sequences using the relatively short-period igniter sequence. The mobile station (MS) performs the synchronization in a manner that it synchronizes the igniter sequence, detects the state samples carried thereon, comparing the state samples with samples of the SRG provided in the mobile station itself, and sequentially corrects the state of its SRG according to a result of comparison. Thus, the DSA technique is referred to the high-speed scrambling sequence acquisition method through the comparison-correction of the SRG state samples.
By using the DSA technique, an effective high-speed synchronization performance can be obtained for both the cell synchronous system and the cell asynchronous system. However, such performance is limited to the case that the signal-to-noise ratio (SNR) for the channel is in a high or properly low level.
Specifically, in case of using the DSA technique, a superior high-speed acquisition performance can be maintained down to the properly low-leveled SNR for the channel, but the DSA technique suffers the abrupt deterioration of performance in the environment that the SNR becomes very low due to a fading or shadowing effect or in case that errors of the state sample detection are frequent due to the clock frequency errors between the base station and the mobile station (MS). Accordingly, a new complementary counter plan is required to maintain the superior high-speed acquisition even in the inferior environment as described above.
Accordingly, the present invention is directed to a high-speed cell searching apparatus for a communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a high-speed cell searching apparatus for a communication system suitable for more rapidly discriminating and acquiring PN sequences having a long period.
Another object of the present invention is to provide a high-speed cell searching apparatus for a communication system which can perform a high-speed acquisition of long-period scrambling sequences used in a W-CDMA system even in a inferior channel environment.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention applies the DSA technique based on the state estimation to the code acquisition process for sequence synchronization in an asynchronous DS/CDMA system. Also, the present invention further employs the comparison-correction-based synchronization and the correlation for the state symbols according to the DSA technique in a DS/CDMA cellular system.
The high-speed cell searching apparatus according to the present invention includes a DSA spreading section and a sample spreading section provided in a base station, and a sample despreading section and a DSA despreading section provided in a mobile station.
The DSA spreading section produces scrambling sequences in a complex form using at least two first main sequences, spreads and scrambles user data using the scrambling sequences, and samples respective states of the at least two first main sequences.
The sample spreading section modulates respective state sample values outputted from the DSA spreading section into code symbols, and spreads the modulated symbols using at least two first igniter sequences.
The sample despreading section produces at least two second igniter sequences, and despreads and demodulates the input state signals using the second igniter sequences.
The DSA despreading section produces at least two second main sequences, synchronizes the second main sequences and the first main sequences of the DSA spreading section using the state samples outputted from the sample despreading section, and despreads and descrambles the spread user data by the complex scrambling sequences produced using the synchronized second main sequences.
Preferably, the period of the igniter sequences is relatively shorter than the period of the main sequences.
Preferably, the base station transmits primary synchronization code (PSC) commonly used in all the base stations, and the mobile station is selectively provided with a matched filter that takes correlation for the primary synchronization code (PSC) received from the base station for the igniter sequence acquisition.
Preferably, the sample spreading section can selectively perform a quadrature phase shift encoding instead of a differential phase shift encoding, and accordingly the sample despreading section can selectively perform a quadrature phase shift decoding instead of a differential phase shift decoding.
In another aspect of the present invention, there is provided a high-speed cell searching method for a communication system comprising the steps of a base station producing at least two first main sequences and modulating state sample values for states of the respective main sequences, the base station spreading and transmitting modulated state symbols with first igniter sequences, a mobile station producing at least two second igniter sequences, the mobile station demodulating the state symbols received from the base station using the second igniter sequences, the mobile station producing at least two second main sequences, the mobile station comparing state samples sequentially obtained by demodulation with state samples for respective states of the second main sequences, and the mobile station performing repeated correction for the respective states of the second main sequences according to a result of comparison.
Preferably, the high-speed cell searching method for a communication system according to the present invention further comprises the steps of storing for a predetermined frame period soft-decision state symbols obtained by the demodulation in preparation for a case that a state of the first main sequences of the base station is not synchronized with a state of the second main sequences of the mobile station through the step of comparison and repeated correction, and acquiring the first main sequences of the base station by performing correlation with respect to the stored soft-decision state symbols.